Apyrases (Ecto-ATP diphosphohydrolases) constitute a group of enzymes catalyzing metabolism of ATP to ADP and ADP to AMP. The first known human apyrase, CD39, was originally identified as a cell-surface protein on activated lymphocytes and endothelial cells. Both the in vitro and in vivo studies clearly indicated that CD39 represents an important apyrase in cardiovascular health by regulating levels of ADP. For example, apyrase is known to inhibit platelet aggregation by metabolizing extracellular ADP. Different from clopidogrel (Plavix) strategies that irreversibly bind to ADP receptor on the platelet, human apyrase does not damage the platelets per se or interfere with normal platelet function providing a safer approach to patients with excessive platelet activation.
Among the known human CD39 family, CD39L3 is known as an ecto-apyrase (ecto-ATPDase) with biochemical activity between CD39 (ecto-ATDP′ase) and CD39L1 (ecto-ATP′ase). Smith and Kirley (Biochemica et Biophysica Acta (1998) 1386:65-78) determined CD39L3 is found primarily in human brain tissue.
Specifically human CD39L3 is a 529 amino acid protein shown in SEQ ID NO:1 with a predicted molecular weight of 59132.42 Daltons. The isoelectric point of CD39L3 is 6.233. There are seven putative glycosylation sites and 13 cysteine residues. Based on SEQ ID NO:1, the N-terminal 43 residues and C-terminal 44 residues are considered to be part of a transmembrane domain. The catalytic core of the enzyme roughly resides between amino acid 44 through amino acid 238, and soluble forms of this protein and related apyrases containing these residues have been prepared and described by Chen, et al. (U.S. Pat. No. 7,247,300). Additionally, substituting a glycine for an arginine at residue 67 and/or an arginine for a threonine at residue 69 is shown to confer additional desired properties including enhanced ADP'ase activity where the residue number refers to the wild-type human CD39L3 shown as SEQ ID NO:1, as described in U.S. Pat. No. 7,390,485.
ProtParam analysis shows that both CD39L3 and CD39 are composed of about 520 amino acids with the pI of about 6.0. CD39L3 and CD39 also share similar amino acid compositions to each other and common structural motifs including about 440 amino acid residues of the extracellular ATP/ADPase portion that resides between the N- and C-terminal transmembrane regions. Although CD39L3 is found in chromosome 3 and CD39 in chromosome 10, their overall intron and exon structures are identical with 10 exons each.
Bioinformatics analysis suggests that CD39L3 is a brain specific isozyme or isoenzyme of CD39. Isozymes or isoenzymes may not have the same regulatory properties of their respective counterpart, but rather have adjusted their enzymatic properties to be optimal for the precise environment to which they are subjected. Northern blot studies showed CD39L3 is highly expressed in brain and kidney, while CD39 is expressed in placenta and spleen. The analysis suggests that expression of the isoenzyme CD39L3 in human brain complements the activity of CD39 as the key thromboregulator.
The present invention provides a new class of apyrases compounds and preparations thereof with improved therapeutic properties such as longer half-life, higher stability, or higher solubility, or higher purity. Methods of making such improved preparations at a high concentration in a form that can be readily purified to substantial homogeneity are also disclosed.